sweep: group pinnable and non-pinnable inputs separately

[morehouse]

The sweeper currently has no concept of pinnable and unpinnable inputs, so HTLC-Timeout (pinnable) and HTLC-Success (not pinnable) inputs can be aggregated into the same transaction if they have the same deadline. As a result, the HTLC-Success inputs effectively become pinnable too.

Another example of this is expired anchors (pinnable) being aggregated with HTLC-Success (not pinnable), though this could also be solved by removing expired anchor sweeping.

I think @yyforyongyu's proposed "sweeper groups" would be all we need to properly implement this.